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If the backside of the hill is steep, you will be wasting all the potential energy in engine braking and air resistance. You would do better if there was no hill at all. Consider three scenarios: Firstly a perfectly flat drive. Second: You drive up a 2% grade and coast down a 2% grade which allows you to EOC at 55 MPH. Third: you drive up a 2% grade for and then down a 4% grade which accelerates you to 75 mph before the air resistance becomes too much.
In the second scenario very little extra fuel is used (if any), but in the third scenario you are essentially buring the extra gas to go 75 on the way up and there is no way around it. Bottom line, the steeper the hill the worse you FE.
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